1. Field of the Invention
The invention relates generally to electric submersible pump assemblies used for hydrocarbon production recovery. In particular aspects, the invention relates to systems for limiting oil leakage from downhole motors used in electric submersible pump assemblies.
2. Description of the Related Art
A typical electrical submersible pump (ESP) system includes a pump that is driven by a motor. Because the ESP system may be disposed at great depths and are inaccessible at this time, the motors are designed to operate for a long period of time without maintenance. Motor oil is used to help lubricate the motor and to dissipate the heat the motor generates during operation. A seal section is used between the motor and pump to isolate the clean motor oil from the wellbore fluid. This seal section also provides for volume change of the motor oil during operation due to changes in pressure and temperature. By allowing the volume of motor oil to change, the internal motor pressure is equalized with the wellbore annulus pressure. A shaft in the seal section transmits torque from the motor to the pump.
Controlling leakage around rotating shafts is often achieved by utilizing mechanical seals. Some leakage will always occur across the faces of a mechanical seal in operation. By design, a small amount of leakage of clean motor oil through the seal is desired to lubricate the faces of the seal. This can be accomplished by having a greater internal oil pressure than external. If the external fluid pressure is higher, leakage will be driven in the other direction, causing external fluid to contaminate the motor oil through the mechanical seal. Over time, as the seal starts to wear, the leakage rate increases and can become problematic to the operation of the equipment. If the seal is not properly lubricated, the wear will be accelerated. Mechanical seal flush plans of various types are used to keep mechanical seals working properly by maintaining proper operating conditions of the seals in order to maximize run life and control leakage. Employing flush plans is relatively straightforward in applications where the mechanical seals are accessible, but it becomes more difficult with rotating equipment that is installed in remote locations, such as downhole pump applications.